Conventional dual action switch assemblies are used, for example, in certain vehicles, such as large load-containing trucks, which are equipped with high and low range rear axle gears. By shifting the rear axle gears from one range to the other the operator may multiply the number of available gears such that the driving forces applied to the wheels are selectively adapted to varying road resistance conditions. The mechanical shifting of the rear axle gears may be accomplished by the action of an axle shift fork device which is controlled by an electric shift unit. The electric shift unit includes a reversible D.C. electric motor, a drive screw assembly and a switch assembly. A conventional switch assembly includes a base carrying two spaced pairs of contacts. Each pair has a fixed contact and a movable contact, normally spring biased into contact with one another. A drive pin, actuated by a reversible motor, is arranged to separate one of the pairs of contacts when the pin is driven in one direction, and to separate the other pair of contacts when driven in the other direction. The separation of the contacts stops the motor. In this way the drive pin may be moved to either of two limiting positions, depending on the direction of rotation of the reversible motor, and thereby cause the desired gear shifting.
However, in such a system, vibration, inertia or other effects may cause the opened contacts to close momentarily which will restart the motor in a direction to open the contacts. Such on-off cycling of the motor may be repeated indefinitely, and cause arcing, resulting in contact erosion and high heat generation in the switch assembly. This in turn may fuse contacts, causing the motor to operate beyond its desired limit and possibly to stall and burn out. As a result of such on-off cycling and arcing, there may be failure of the drive screw assembly, electric motor or switch; requiring frequent replacement.